Combination coatings



Oct. 1, 1963 E. LAVIN ETAL 3,105,775

COMBINATION commas Filed April 5. 1961 FIG.|'

' CONDUCTOR POLYVINYL FORMAL ENAMEL over POLYIMIDE ENAMEL FIG.2

CONDUCTOR IIIIIIIIIIIIIIIIII IIIIIIIIIIA POLYIMIDE ENAMEL over POLYVINYL FORMAL ENAMEL FIG. 3

POLYIMIDE ENAMEL over POLYVINYL FORMALENAMEL over POLYIMIDE ENAMEL EDWARD LAVlN ALBERT H. MARKHART CHARLES E HUNT I INVENTOR ATTORNEY United States Patent 3,105,775 CGMBiNATIGN COATINGS Edward Lavin, Longmeadow, Albert H. Marithant, Wilbraham, and Charles F. Hunt, Springfield, Mass., as-

signors to Shawinigan Resins Corporation, Springfield,

Mass, a corporation of Massachusetts Filed Apr. 3, 1961, Ser. No. 189,015 11 Claims. (Cl. 117-232) This invention relates to enameled wires; more particularly, it relates to wires coated with multiple layers of cured polyvinyl acetal resin compositions and polyimide resin compositions.

'Magnet wires coated with compositions based on polyvinyl formal are well known. They are generally. used at operating temperatures not exceeding 105 C. and, for this reason, are considered to be class A wires, in accord with the prevailing electrical insulation code. The delicate balance of excellent mechanical, chemical and electrical properties possessed by these wires and their relatively low cost are certainly important factors contributing to the wide use of these polyvinyl formal wires in the electrical industry.

It is well established, on the other hand, that the output of motors and generators is increased considerably when the density of the current in the magnet wire of these machines is raised. Unfortunately greater current density creates higher operating temperatures, and that shortens the life of class A wire. Yet, if one keeps in mind the well balanced physical properties of polyvinyl acetal coated wire, its low cost, its general acceptance, the large investment of the wire coating industry in equipment designed to produce it, and the shortcomings of other classes of wires that have been proposed as substitutes, the desirability of upgrading class A polyvinyl acetal magnet wire to higher classifications while maintaining its other present acceptable properties, becomes evident.

A method commonly used to upgrade the thermal life of polyvinyl acetal coatings consists in impregnating the ready to use wire winding with special varnishes. However, because of difierences in chemical composition, solubility and viscosity between varnishes and enamels, the varnishes do not achieve the improvement sought by this invention and, moreover, suifer from the inconvenience that they must be applied after motor and generator coils are wound.

It is therefore an object of this invention to upgrade the thermal life of polyvinyl acetal magnet wire so that it will perform efficiently at operating temperatures up to 130 C. (class B) and up to 155 C. (class F). Another object is to produce organic class B and class F wire coatings which can resist hydrolysis, are not bulky and possess good abrasion resistance for machine wind- These and other objects are attained by applying a thin layer of certain aromatic polyamide enamels over and/ or under a coat of polyvinyl acetal or modified polyvinyl acetal enamel.

The following examples will serve to illustrate but not to limit this invention. For instance, although it has been found practical and convenient to efiect all the polyvinyl acetal coatings with well known standard preparations or mixtures of components available commercially, it must not be concluded that the invention is thus circumscribed.

In the accompanying drawing, longitudinal sectional views of wires (conductors) insulated in the manner of this invention are shown. FIGURE 1 illustrates a wire coated with two layers of polyimide enamel and four layers of polyvinyl formal enamel; the polyimide layers are next to the metal. In this and subsequent figures, the thickness of the insulation coatings is scaled to show approximately the number of layers of each type of coating. in FIGURE'Z, the order of superposition of enamels has been reversed so that the polyvinyl formal enamel is now next to the metal. In FIGURE 3, the conductor is covered with four layers of polyvinylformal sandwiched between two layers of polyimide.

EXAMPLES 1-7 Pour enamels containing a polyvinyl acetal resin have been employed along with one laboratory synthesized polyamide enamel and a commercially available enamel chemically similar in nature. The compositions of the polyvinyl :acetal enamels are shown, in parts by weight, in Table I. The polyamide enamels will be described subsequently.

Both these components and enamelsrare well known to the trade.

The polyvinyl formal resin employed contains 10.5% acetate groups calculated as polyvinyl acetate, 6% 'hydroxyl groups calculated as polyvinyl alcohol, and 83.5% formal groups calculated by difference as polyvinyl formal. The phenolic resin is a soluble, fusible, heathardenable reaction product of 100 parts cresol, 60 parts formalin, and 3.2 parts ethanolamine, dissolved in commercial cresylic acid. The polyurethane is the phenol blocked reaction product of one mo-l trimethylol propane with three mols of a mixture containing about 2,4- and about 20% 2,6-tolylene diisocyanates. The melamine-formaldehyde resin is a relatively low molecular weight, butylated, internally plas-ticized condensation product of 1 mol melamine with 3.5 mols formaldehyde and 0.5 mol para-toluene sulfonamide. Finally, the last two components employed in the preparation of these enamels are commercial cresylic acid and naphtha. Little need ,be said about enamels A to D themselves; they will be easily recognized by workers the field.

One of the polyamides employed in the examples, enamel E, is the condensation product of pyromellitic dianhydride with 4,4'-oxydianiline. The oxyariiline was recrystallized from ethyl alcohol-dimethyl acetamide, and technical pyromellitic dianhydride (86.7%) was purified by heating at 250 C. for two hours at 80 mm. pressure. The resulting pyromellitic dianhydride, 14.8 g., was slurried in 71 ml. dry dimethyl-acetamide in a 3-neck, round-bottom flask fitted with a thermometer and an air condenser, protected with a calcium chloride drying tube. A solution of 13.4 g. oxydianiline in 71 ml. dry N-methylpyrrolidone was added. The reaction mixture was held at 50 C. with stirring for two hours. The polyamide The scrape abrasion test-This test consists essentially in scraping repeatedly the insulated wire sample with a steel needle loaded with a set weight which is varied according to the build of the Wire. The tester solution thus obtained was applied on wire. Suflice it 5 records the number of strokes and shuts off when contact to mention at this stage that the polyamide, after it has is made between the needle and the bared wire. Insulabeen applied to the wire, is converted in situ to a tion which withstands an average of 30 strokes in six polyamide structure during the subsequent processing of different places is considered standard provided that no the wire. one place wears down with less than 15 strokes.

The other polyamide enamel used in the examples (F) The hydrolysis resistance test.In this test, entwined is a commercial product marketed under the brand name wire pairs are heated at 150 C. in sealed glass tubes ML; it consists essentially of a polyamide chemically containing 0.5 ml. water. At definite intervals, a pair similar to that in enamel E, dissolved as a by is taken out and is tested under increasing electrical weight solution in a 1:1 mixture of N-methylpyrrolidone potential until the insulation breaks down. Results are and dimethylacetamide and having a viscosity of about 15 reported as the period, in hours, of exposure to the con: 4800 cps. at 25 C. ditions of the test, that the wire can withstand before it The enamels were applied to number 18 (0.0403 inch) breaks down under a potential of one-kilovolt applied copper wire by conventionalwire coating means, each for one second. i layer of enamel being cured by passing the dipped wire Table II shows the significant properties of the control through a vertical oven 12 feet high with a set cure tem 20 wires covered with standard enamels at optimum cure. perature of 350 C. at a constant rate of speed selected These values will serve as a basis for evaluation of the from the range of 12 to 19 feet of wire per minute. new insulations of the later examples.

Table II 1 kv.-life (hrs) Hydroly- Enamel Build Abrasion sis Re- (mils) (strokes) slstanee 180 0. 200 0. 225 0. 250C 300C (hours) 3 43 21s 12s 25 10 4 144 3 63 152 113 2a 12 a. 5 144 3 ca 439 15s 34 1a. 3 144 3 78 435 180 1s 3. 5 1s4 1 2 2, 000 2,000 300 41 2s 10 1 1 2,000 2, 000 54.4 50 27 16 3 s 2, 000 2, 000 2,000 1,600 058 10 A split applicator was employed so that the desired (The values accompanied by a superscript are not final number of different coats could be put on in one conin that at the time indicated by the figures, e.g. 2000 hours tinuous operation. for enamel F at 200 C. a number of samples repre- Control wires were prepared with each enamel by 40 sented by the superscript were still resisting the condiapplying six successive coats of the enamels to the bare tions of the test. It will later become clear that these wire. Since each coat has a thickness of approximately incomplete values do not detract from the understanding 0.25 mil, the resulting wires were thus given a heavy of the invention.) The results, as they are, certainly build, the insulated wires having a diameter approxidemonstrate the excellent abrasion and hydrolysis resistmately 3.0 mils larger than that of the bare original ance of polyvinyl formal enamels (A to D) and the wires. In the case of enamels E and F, the polyamides, 45 excellent thermal stability of the polyimide insulations a two coat control wire was also prepared, having a (E and F). build of approximately 1 mil. EXAMPLES 8- 13 The testing of the enameled wires was done by stand- 7 and procedures, Althgugh n m h i l, physical Wires were then prepared with insulation consisting and electrical properties of the control wires and of the of four coats of polyvinyl formal resin over two coats new insulated wires were tested, the descriptions of the 0f Polyimide enamel as illustrated y FIGURE The testing procedures will be limited to those measuring significant properties of these new coatings are sumthe properties in which significant changes were observed, marized in the following table.

Table III 1 kWh-lite (hours) Hydrol- Build Abrasion ysis Enamel (mils) (strokes) resistance 180 0 200 0. 225 0. 250 0. 300 0 (hours) A on 11---. 2+1 31 2.000 2,000 2, 000 620 112 144 0 on E. 2+1 40 2.000 2.000 2,000 880 124 144 Aon F- 2+1 '34 2,000 2,000 1, 050 547 145 144 V B on F- 2+1 as 2,000 2,000 1,900= 715 117 144 c on F 2+1 50 2,000 2, 000 2,000 901 152 144 D on F 2+1 2.000 2,000 2,000 009 117 144 namely one-kilovolt life, abrasion resistance and resistance A comparison of these results with those of the control to hydrolysis. wires of Table II will quickly indicate that, although the 1 kilovolt-life test.The 1 kv.-life test was made in regular six coat (3 mil build) polyimide wire has a theraccordance with the provisions of the American Institute mal life superior to any of the above combination enof Electrical Engineers specifications No. 57, dated Octo- 70 amels, two coats of enamels E or F placed under four ber 1955. The test is a measure of the period for which coats of polyvinyl formal enamel will form insulation a coating can be exposed at the particular temperature that is far superior than that provided by twocoats of indicated before it will fail as electrical insulation upon enamel F alone. A comparison of the thermal life of the application of 1000 volts for one second to the sample. combination coatings with that of the one mil polyimide Ten samples are used in each test. coatings at temperature 225 C. and above shows that the improvement in the polyvinyl formal enamel thermal life is not due merely to the presence of the polyimide in the combination coatings, since at those temperatures, the thermal lives of the combination coat ngs are longer 6 depending upon the extent of the hydrolysis and the acetalization reactions; the preferred resins will be those containing on a weight basis from about 1 to about 35% ester groups calculated as polyvinyl ester, from about 3 than those of either the polyvinyl formal coating and 5 to about hydroxyl groups calculated as polyvinyl the one mil pol-yimide coatings alone. It should also alcohol, the balance being substantially aldehyde acetal. be noted that in abrasion resistance and in hydrolysis re- In the commercial polyvinyl formals which were used sistance, the superiority of the combination coatings exin the examples, the ester groups were acetate groups. tends also to the 3 mil (six coat) polyimide enamel. While formaldehyde is preferred as an acetalizing agent because of its greater reactivity, the process may be EXAMPLES 14-15 10 carried out with acetaldehyde, propionaldehyde, butyral- In these examples, the order of application of the endehyde and mixtures thereof. Higher aliphatic aldeamels to the wires was reversed so that the insulation on hydes, as well as aromatic aldehydes, may also be emthe wires now consisted of two coats of polyimide enamel ployed. over four coats of polyvinyl formal enamel, as illustrated 1 The phenol aldehyde resins used in these compositions by FIGURE 2. The following results were noted: are soluble, heat-hardenable condensates of a phenol Table IV 1 kv.-1ife (hours) Enamels Build Abrasion (mils) (strokes) Fon o 1+2 11 2,000 2, 000 2, 000 1,000 33s FonD 1+2 22 2,000 2,000 2,000 1, e00 250 An even greater advance in thermal life of polyvinyl and an aldehyde, as revealed in U.S.P. 2,307,588. They formal enamel is thus obtained by coating the polyimide are generally prepared by reacting 1 mol of a phenol over the enamels C and D rather than under them. with from 0.7 to 2 mols of a lower aldehyde under EXAMPLES 1647 alkaline conditions. Acetaldehyde, propionaldehyde and butyraldehyde may be used as Well as others, but rormal- The Combination efielt Obtained y Overcoming dehyde is usually preferred because of its greater reacamel F is SO great in fact that only 0116 coat Of it OVCI' tivity A Variety 0f phenols may be used including five Coats of P y y formal enamel is considerably monohydric phenols such as phenol, cresols, xylenols, superior to two coat enamel .F insulation alone, as will ethylphenol, 1 phenol, other alkyl phenols or become evident when the results in the next table are mixtures h f comPal'ed With the pp p Ones in Table Usable polyurethanes are polyisocyanates blocked Table V with organic compounds containing at least one reactive hydrogen atom. These blocking agents must split off 1 k at enamel cure temperatures in order to provide free isov.-l1fe (hours) Enamel 40 2:11 :22: mgerloups to crosslmk the resinous compositions of 180 200 225 3 g Suitable reactive hydrogen compounds include phenols, such as phenol, cresol, xylenols, etc., secondary aromatic Fon'C 0. 5+ 2.5 28 2,000 1,800 1, 900 792 so amines, monoand polyfunctional alcohols, amines, lac- CI1F 23 11100 508 265 29 tams, enols, and mixtures thereof. The preferred blocking agents are compounds in which an hydroxyl group It is also obvious from these results that a single enamel i h d to an aromatic i F Undercoat With five P y y fm1$ rfisin Coats is The simplest class of useful polyisocyanates can be not as desirable as a single enamel F overcoat. re resented by the following formula:

However, when a sandwich type of insulation was put on a wire, with four coats of polyvinyl formal enamel R( 'N C:O)n placed between two non-contiguous coats of polyimide where R represents a member of the class consisting of as shown in FIGURE 3, a thermal life was obtained aliphatic hydrocarbons containing up to 8 carbon atoms, which has been shown to be intermediate between two aromatic hydrocarbons containing up to 13 carbon atoms, contiguous enamel F undercoats and two contiguous alicyclic hydrocarbons containing up to 6 carbon atoms, enamel F overcoats. and alkyl-aryl substitutes thereof, and n is an integer in summary then, it is shown conclusively in the exfrom 2 to 4. Suitable polyisocyanates include compounds amples that the thermal life of polyvinyl acetal resin such as phenylene diisocyanates, diphenylene diisocyainsulations is sensibly lengthened when they are used nates, tolylene diisocyanates, naphthylene diisocyanates, in conjunction with thinner layers of a polyimide enamel, diphenylmethane diisocyanates, cyclohexane diisocythe excellent abrasion and hydrolysis resistance of the anates, ethylene diisocyanates, tetramethylene diisoformer enamels being retained in the process. cyanates, hexamethylene diisocyanates, polyaryl polyiso- It is evident, of course, that many widely diilerent cyanates, trimers of polyisocyanates, polyisocyanates embodiments of this invention other than those provided which are the reaction products of diisocyanatcs or triin the examples may be made by persons. skilled in the isocyanates with polyhydric alcohols and the like, and art without departing from the scope and the spirit theremixtures, trimers and isomers thereof. of. The preferred polyurethanes are produced from mono- For instance, the polyvinyl acetals that may be used mers or trimers of aromatic diisocyanates which are fully in this invention are obtained by the acctalization of blocked in order to advantageously avoid premature curpolyvinyl esters, partially hydrolyzed polyvinyl esters, ing and yet obtain the rapid, uniform and complete curand fully hydrolyzed polyvinyl esters, according to lug required for good solvent resistance, heat stability and methods well known in the art, such as those found in the numerous other properties needed for satisfactory U.S. Reissue No. 20,430 to Morrison et al. Polyvinyl electrical insulation. 7 acetals normally contain a certain number of hydroxyl This preferred class of polyurethanes is made of the groups and may contain a certain number of ester groups blocked reaction products of a polyhydric alcohol with an arylene diisocyanate. The polyhydric alcohols are in general limited to compounds containing not more than 16 carbon atoms and when used in wire enamels should preferably contain not more than 10 carbon atoms. Examples of these polyhydric alcohols are ethylene glycol, propylene glycol, glycerol, trirnethylol propane, pentaerythritol, hexane-triols, etc. The class of polyurethanes is illustrated by the general formula OIEIII HO where R is an aromatic hydrocarbon radical containing up to 13 carbon atoms, or an alkyl substitute thereof. The isocyanate groups are blocked with a reactive hydrogen containing compound such as phenol.

The melamine resins which can be used in the present wire enamel compositions can be selected from the general class of resinous aldehyde condensation products of melamine which are soluble in the organic liquids employed as solvents for the resinous components of the enamel. The useful melamine compounds include such derivatives of melamine as melam and melem. The alde hyde condensation products are well known and may be formed by reacting from l-6 mols of the aldehyde with 1 mol of melamine. The solubility of the aldehydemelamine condensation product is generally obtained by further reacting the condensation product with an alcohol or by co-condensing the melamine and aldehyde in the presence of an alcohol.

The usable aldehydes are aliphatic, aromatic cyclic and heterocyclic aldehydes including formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, isobutyraldehyde, valeraldehyde, octaldehyde, benzaldehyde, cinnamaldehyde, cyclohexanone, furfural, etc.

The alcohols which may be used include aliphatic, cycloaliphatie, aromatic, nitro and amino alcohols such as methanol, ethanol, propanol, isopropanol, butanol, isobutanol, pentanols, octanols, lauryl alcohol, cetyl alcohol, stearyl alcohol, cyclohexanol, benzyl alcohol, cinnamyl alcohol, allyl alcohol, 2-nitro-l-butanol, 2-nitro-2-methyll-propanol, 2-nitro-2-methyl-1,3-propane diol, 2-nitro-2- ethyl-1,3-propane diol, tris(hydroxymethyl)nitromethane, 2 amino 1 butanol, 2-arnino-2-methyl-l-propanol, 2- amino-2-methyl-1,3-propane diol, 2-amino-2-ethyl-l,3- propane diol, tris(hydroxymethyl) aminomethane etc. Mixtures of two or more alcohols may be used if desired. The amounts of alcohol reacted are generally equal to or in excess of the formaldehyde on a molar ratio.

The preferred melamine resins are the further reaction products of the melamine aldehyde and alcohol reactants with an aryl sulfonamide. These products are also well known and may be obtained by co-condensation of all the reactants named, as taught in US. 2,508,875, which is hereby incorporated by reference. The useful aryl sulfonamides include benzene sulfonamide and the ring substituted derivatives thereof such as toluene sulfor1 amides, chlorobenzene sulfonarnides, nitrobenzene sulfonamides, etc. i

For reasons of economy and availability, it is preferred to use the co-condensation products of melamine, toluene sulfonamide, formaldehyde and butanol. The proportions of reactants may be varied between the limits of 1 mol of melamine to from 0.1 to 1.0 of toluenesulfom amide and from 1 to 6 or more mols of formaldehyde. An excess of the formaldehyde may be used. The toluene sulfonamide may be any of the isomeric ortho, meta or para derivatives or it may be a mixture of two or more of the isomers.

The polyamides useful in this invention are the con-- densation products of tetracarboxylic aromatic acids with diamines and may be illustrated by the recurring structural unit These polymers are soluble, and can be converted in situ, as mentioned earlier, by further heating into polyimide structures that are extremely resistant to solvents:

diphenyl methane, 2,2-diphenyl propane, diphenyl ether,

diphenyl sulfide and diphenyl sulfone. The R of the formulae stands for a bivalent radical of either benzene, naphthalene, biphenyl, diphenyl ether, ditolyletheryd-iphenyl sulfide, diphenyl sulfone, diphenyl methane, 2,2- diphenyl propane, benzophenone, or from a low molecular weight saturated aliphatic hydrocarbon containing not more than siX carbon atoms.

The preferred class of polyamides is limited to the condensation products of pyromellitic dianhydride with a diarnine characterized by a lack of aliphatic hydrogen atoms and by the possession of a flexible linkage such 'as the ether group of 4,4'-oxydianiline. These polyamides should preferably be such that they show a viscosity ranging from about 1300 centipoises to 5000 centipoises for a 15% by weight solution in 1:1 dimethylacetamide and N-methylpyrrolidone at 25 C.

Some of the resinous coatings that may be used in this invention are the solid reaction products, cured at 250 to 450 C. of 100 parts by weight of a polyvinyl acetal resin, 2.5 to 100 parts of a phenol aldehyde resin, and

when desired, 10 to 200 parts of a polyurethane with or without 0.5 to 20 parts of a melamine resin. Alternately the phenol-aldehyde resin may be excluded altogether from such compositions. The preferred balance of the properties required for insulated wire is obtained by the application of polyvinyl acetal resins containing, for each 100 parts of polyvinyl acetal, 5 to parts of a phenol aldehyde resin, 5 to 80 parts of a polyurethane and 0.5 to 20 parts of a melamine resin, and of a polyamide resin made from pyromellitic dianhydride and 4,4-oxydiani- The preferred number of coats of resins are four and two for the acetal resins and the polyamide resin respectively for heavy build magnet wire. This invention is not bound by these optimal figures, however. Under certain circumstances, wires with satisfactory properties can be made with an insulation cover of any thickness between 0.1 to 4 mils, this up to a point being a function of the wire diameter and the viscosity of the enamels, applied in 2 to 14 layers. As to the particular number of coats of a particular enamel, no restriction is intended to the four and two arrangement. Combinations of five coatsof polyvinyl acetal resin and one coat of polyamide, for instance, have shown quite acceptable properties. As to the order of application of the resins, the examples have shown that while an outside covering of polyamide gives the best thermal results, the use of the polyamide as an undercoat to the polyvinyl acetal resin also achieves a great improvement in the significant wire properties.

These combinations of enamels may be used on any size of wire, on a variety of metals, and on other materials. Other non-electrical uses of this coating process are indicated where chemical resistance, temperature stability, smoothness, toughness, adherence to metal, resistance to abrasion or resistance to solvents is required of the finished coating.

Various other materials such as fillers, plasticizers, coloring agents, etc., as well as minor amounts of other resins such as polyesters and epoxies, may be incorporated in these enamels as is conventional in the art.

Other applications will readily suggest themselves to the many skilled in the art.

What we claim is:

1. A metal conductor coated with multiple layers of a cured polyvinyl acetal resin composition and of a polyimide composition comprising the condensation product of an aromatic tetracarboxylic acid with a diamino compound selected from the group consisting of aromatic diamines containing from 6 to 16 carbon atoms and saturated aliphatic diamines containing up to 6 carbon atoms.

2. A metal conductor coated with insulation of a thickness of 0.1 to 4.0 mils consisting of 2 to 14 layers of the resins of claim 1.

3. An insulated conductor comprising a metallic element coated with multiple layers of (A) a resinous composition comprising, in parts by weight,

(1) 100 parts of the polyvinyl acetal of a satu rated aliphatic aldehyde and (2) 5 to 80 parts of a soluble, heat-hardenable alkylphenol-formaldehyde resin (B) and of another resinous composition comprising the condensation product of an aromatic tetracarboxylic acid with a diamino compound selectedfrom the group consisting of aromatic diamines containing from 6 to '16 carbon atoms and saturated aliphatic diamines containing up to 6 carbon atoms.

4. The conductor of claim 3 wherein the polyvinyl acetal is polyvinyl formal.

5. The conductor of claim 3 wherein thepolyvinyl acetal composition is modified by the inclusion of 10 to 80 parts of a blocked organic polyisocyanate.

6. The conductor of claim 5 wherein the blocked polyisocyanate is the phenolic adduct of the reaction product of about one mol trimethylol propane with about three mols tolylene diisocyanate.

7. The conductor of claim 5 wherein the polyvinyl acetal composition is further modified by the inclusion of 0.5 to 20 parts of a melamine formaldehyde resin.

8. A process for producing a wire coated with 0.1 to 4.0 mil thick covering consisting of about 2 to 14 layers of insoluble and infusible resins, separately dried and cured at 250 to 450 C., originally applied by passing the wire through two liquid compositions comprising essentially, in parts by weight,

10 (A) in one of the compositions,

(1) 100 parts of a polyvinyl acetal resin obtained by the partial acetalization of polyvinyl alcohol with a member of the group consisting of formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde and mixtures thereof. a

(2) 5 to parts of a soluble, heat-hardenable phenol aldehyde resin obtained by condensing one mol of a phenol selected from the group consisting of phenol, cresol, xylenol, and mixtures thereof, with from 0.75 to 2.0 mols of formaldehyde, under alkaline conditions,

(3) 10 to 80 parts of a blocked organic polyisocyanate adduct of the general formula class consisting of phenyl and alkyl phenylgroups, said alkyl groups containing 1 to 6 carbon atoms, where m is an integer greater than 1 but not greater than n, and where n is an integer from 2 to 10;

A (4) 0.5 to 20 parts of the co-condensation prod nets of one mol melamine with from 0.1 to

' 1.0 mol of a toluene sulfonamide and from 1 to 6 or more mols formaldehyde, in the presence of butanol;

(B) and in the other composition, the polyamide condensation product of pyromellitic dianhydride with a diamino compound selected from the group consisting of aromatic diamines containing from 6 to 16 carbon atoms and saturated aliphatic diamines containing up to 6 carbon atoms. 9. A process for producing a wire coated with 0.1 to 4 mil thick covering consisting of about 2 to 14 layers of insoluble and infusible resins, separately dried and cured at 250450 C., originally applied by passing the wire through two liquid compositions consisting essentially of, in parts by weight,

(A) for one of the compositions (1) parts of polyvinyl formal containing about 10.5% acetate groups calculated as polyvinyl acetate, about 6% hydroxyl groups calculated as polyvinyl alcohol, and about 83.5% formal groups calculated by difference as polyvinyl formal,

(2) 5 to 45 parts of a soluble, fusible, heat-hardenable reaction product of about 100 parts cresol, about 60 parts formalin and about 3.2 parts ethanolamine;

(3) 40 to 65 parts of the phenolic adduct of the reaction product of about one mol trirnethylol propane with about three mols tolylene diisocyanate;

(4) 2.5 to 12.5 parts of the butylated internally plasticized condensation product of about one mol melamine with about 3.5 mols formaldehyde and about 0.5 mol para-toluene sulfonamide;

(5) 200 to 750 parts of cresylic acid (6) 0 to 600 parts of naphtha (B) and for the other composition, the polyamide condensation product of pyromellitic dianhydride with 4,4-oxydianiline dissolved in equal volumes of dimethyl acetamide and N-methylpyrrolidone.

10. A metal conductor coated with insulation of a thickness of about 3 mils consisting of (A) four layers of a resinous composition comprising,

in parts by weight,

(1) 100 parts of a polyvinyl acetal resin obtained by the partial acetalization of polyvinyl alcohol with a member of the consisting of formaldehyde, propionaldehyde, butyraldehyde and mixtures thereof,

(2) to 80 parts of a soluble, heat-hardenable phenol aldehyde resin obtained by condensing one mol of a phenol selected from the group consisting of phenol, cresol, Xylenol, and mixtures, thereof, with from 0.75 to 2.0 mols of formaldehyde under alkaline conditions,

(3) to 80 parts of a blocked organic polyisocyanate adduct of the general formula where R is a member of the class consisting of phenylene, methyl phenylene, dimethyl phenylene, naphthylene and methyl naphthylene groups, where X represents a member of the class consisting of phenyl and alkyl phenyl groups, said alkyl groups containing 1 to .6 carbon atoms, Where m is an integer greater than 1 but not greater than n, and where n is an integer from 2 to 10,

(4) 0.5 to 20 parts of the co-condensation products of one mol melamine with from 0.1 to 1.0

mol of a toluene sulfonarnide and from 1 to 6 or more mols formaldehyde, in the presence of butanol, and

(B) two layers of the polyimide condensation prod uct of pyromellitic dianhydride with a diamino compound selected from the group consisting of aromatic diamines containing from 6 to 16 carbon atoms and saturated aliphatic diamines containing up to 6 carbon atoms.

11. A metal conductor coated with insulation of a thickness of about 3 mils consisting of (A) four layers of a resinous composition comprising, in parts by weight,

(l) 100 parts of a polyvinyl formal containing about 10.5% acetate groups calculated as polyvinyl acetate, about 6% hydroxyl groups calculated as polyvinyl alcohol, and about 83.5%

formal groups calculated by difference as polyvinyl formal,

(2) 5 to parts of a soluble, fusible, heathardenable reaction product of about parts plasticized condensation product of about one.

mol melamine with about 3.5 mols formaldehyde and about 0.5 mol para-toluene sulfonamide; and (B) two layers of the polyimide condensation product of pyromellitic dianhydride with 4,4'-oxydianiline;

References Cited in the file of this patent UNITED STATES PATENTS 2,333,922 Foster Nov. 9, 1943 

1. A METAL CONDUCTOR COATED WITH MULTIPLE LAYERS OF A CURED POLYVINYL ACETYL RESIN COMPOSITION AND OF A POLYIMIDE COMPOSITION COMPRISING THE CONDENSATION PRODUCT OF AN AROMATIC TETRACARBOXYLIC ACID WITH A DIAMINO COMPOUND SELECTED FROM THE GROUP CONSISTING OF AROMATIC DIAMINES CONTAINING FROM 6 TO 16 CARBON ATOMS AND SATURATED ALIPHATIC DIAMINES CONTAINING UP TO 6 CARBON ATOMS. 